Device for casting thin strips of metal between

ABSTRACT

The surface of each roll (2, 3) has a knurling constituted by two series of parallel grooves (5, 6) which are inclined to the axis (XX) of the roll and joining up at the center of the latter so as to form a series of chevrons, the two grooves (5, 6) of each chevron downwardly converging when they are on the side of the roll (2, 3) in contact with the liquid metal. The device is completed by a cage for extracting the cast product at a speed slightly higher than the speed of rotation of the casting rolls (2, 3). This arrangement avoids the formation of longitudinal or transverse cracks on the surface of the cast thin metal strip.

The present invention relates to a device for casting thin strips ofmetal, in particular steel, on a roll or between two cooled parallelrolls rotating about their respective horizontal axes in oppositedirections and disposed at a short distance from each other so as todefine a casting space therebetween.

It has been found that strips of great width cast with these devicesoften have longitudinal surface cracks. The explanation of the formationof these cracks resides in the simultaneous character of the shrinkageof the metal from the start of its surface solidification or freezingand a relative "adherence" of the metal skin to the surface of the rollswhich in fact opposes this shrinkage.

In order to solve this problem and cause these cracks, which affect thequality of the surface of the cast product, to disappear, it has inparticular been proposed to provide surface irregularities in the formof a check pattern on the rolls. However, it has been found that thesecheck patterns are not fully satisfactory.

An object of the invention is therefore to provide a casting device ofthe aforementioned type which is so arranged as to solve the consideredproblem in a satisfactory manner.

According to the invention, the device for casting thin metal strips ischaracterized in that there are formed on the surface of each rollparallel grooves inclined to the axis of the roll and joining up in atransverse plane of the latter so as to form a series of chevrons on thesurface of the roll, the two grooves of each chevron convergingdownwardly in the direction of extraction of the cast strip when thegrooves face toward the casting space.

As the inclination of the grooves is suitably chosen in accordance withthe characteristics of the device and as the shrinkage of the metal uponsolidification occurs in a direction from the two ends of the rolltoward the centre of the latter, it will be understood that thisinclination encourages a slipping of the skin of solidified metal towardthe axially median plane of the roll, which opposes the formation ofcracks.

Advantageously, the device is completed by a cage disposed below therolls and comprising a pair of strip extracting rolls which rotate at aspeed slightly higher than the speed of rotation of the rolls betweenwhich the liquid metal is cast.

This slightly higher speed drives the already solidified strip, whichresults in a slight slipping of the strip in the direction of thegrooves during the solidification on the rolls. This slipping thereforeadvantageously accompanies the transverse shrinkage of the product.

It will be understood that the angle of inclination of the grooves tothe horizontal and the amount by which the speed of the extracting rollsof the cage exceeds the speed of the casting rolls, are suitablydetermined with the aid of a crack detecting system: the persistence ofthe longitudinal cracks signifies that the higher speed is insufficient,while the formation of transverse cracks signifies that the higher speedis excessive and the tension of the strip is therefore excessive.

The invention will now be described with reference to the accompanyingdrawing which illustrates an embodiment thereof by way of anon-limitative example.

IN THE DRAWING

FIG. 1 is a diagrammatic end elevational view of a device for castingthin strips of metal between two rolls equipped with an extracting cagein accordance with a preferred embodiment of the invention.

FIG. 2 is a longitudinal elevational view of one of the rolls of thedevice of FIG. 1 with grooves formed on its surface in accordance withthe invention.

The device shown in the drawings is adapted to cast a thin strip 1 ofsteel between two parallel, horizontal and energetically cooled rolls 2and 3 rotating in opposite directions. These rolls are spaced a slightdistance apart from each other so as to define therebetween a castingspace 4 for receiving a bath of liquid metal from above and allowing themetal to leave the device from below after solidification of the metalin contact with the cold walls of the rolls under the effect of therotation of the latter.

The rolls 2, 3 are rotated about their respective axes XX and cooledinternally by well-known means (not shown). The casting spacetherebetween is supplied with liquid metal, for example by a nozzle 9mounted on the bottom of a distributor (not shown) placed thereabove.

Formed on the surface of each roll 2, 3 is a knurling constituted by twoseries of parallel grooves 5 and 6 which are inclined to the axis XX ofthe roll and join up at the centre of the latter in its mediantransverse plane M so as to form a series of chevrons on the whole ofthe surface of the roll.

A first series of parallel grooves 5 is in this way provided on one halfof each roll 2, 3 starting at one of the end faces of the latter, and asecond series of parallel grooves 6 is provided on the other half of theroll starting at the other end face. The grooves 5 and 6 have the sameinclination A with respect to the axis XX, each pair of grooves 5 and 6constituting a chevron the two grooves of which converge downwardly inthe direction of extraction of the product 1 shown by the arrow 10 whenthey face toward the casting space 4.

The depth of the grooves and their inclination A are suitably chosen inaccordance with other parameters of the device. For example, the angle Amay be of the order of 45° and the depth of the grooves may be aroundone hundred microns or even more.

The device is completed by a cage 7 placed below the casting rolls 2, 3and comprising a pair of rolls 8 for extracting the strip 1 which isconsequently pinched between these two rolls 8. The latter are rotatedby means known per se (not shown) and rotate at a speed which isslightly higher than the speed of rotation of the casting rolls 2 and 3.This higher speed of the extracting rolls 8--which is a few percenthigher than the speed of the casting rolls--and the angle of inclinationA of the grooves 5, 6 relative to the horizontal may be determined withthe aid of a crack detector, as mentioned before.

When the liquid metal 4 starts to solidify or freeze on contact with thecooled surface of the rolls 2, 3 and thus form a skin, it at the sametime undergoes a shrinkage starting at the ends of the roll,symbolically represented by the arrows F. The downward inclination A ofthe grooves 5, 6 therefore encourages this shrinkage of the skin and thedownward slipping of the latter in the direction of the extraction ofthe product resulting from the slightly higher speed of rotation of theextracting rolls 8 relative to the speed the rolls 2, 3. If the amountby which the speed of the rolls 8 exceeds the speed of the casting rolls2,3 and the angle of inclination A of the knurling are suitably chosen,there is observed a practically complete disappearance of thelongitudinal and transverse cracks on the surface of the cast product 1.

As already mentioned, the speed of rotation of the rolls 8 is increasedif the presence of longitudinal cracks is observed, and, on thecontrary, decreased if transverse cracks are observed.

The invention is also applicable to the case where the casting devicehas only a single roll, the liquid metal being cast on the roll from avessel placed at the side of the latter.

Further, the invention is further applicable in the case where the rollsare grooved on only a portion of their width, for example in the centralportion.

What is claimed is:
 1. A device for casting a thin metal strip comprising:at least one roll rotatable about an axis, said at least one roll having a surface; means for supplying molten metal to said surface of said at least one roll when a thin metal strip is to be cast; said at least one roll further including:a plurality of nested, spaced apart, V-shaped chevron-like grooves in said surface of said at least one roll; each V-shaped chevron-like groove having first and second arm portions defining an angle therebetween; and each of said first and second arm portions of each of said substantially V-shaped chevron-like grooves extending away from said angle formed therebetween in a first direction which is opposite to a second direction in which said thin metal strip is to be cast.
 2. The device according to claim 1, wherein the angle defined by said first and second arm portions of each V-shaped chevron-like groove is a given angle, said given angle being the same for all of said plurality of V-shaped chevron-like grooves in said surface.
 3. The device according to claim 2, wherein corresponding first arm portions of said plurality of V-shaped chevron-like grooves are parallel to each other, and corresponding second arm portions of said plurality of V-shaped chevron-like grooves are parallel to each other.
 4. The device according to claim 1 in which said at least one roll is rotatable about a horizontal axis at a preselected speed, the device further comprising:a cage disposed below said at least one roll in said second direction, said cage including:a pair of strip extracting rolls for receiving said metal strip cast by said at least one roll, said strip extracting rolls being rotatable at a predetermined speed slightly higher than said preselected speed.
 5. The device according to claim 1, wherein each of said V-shaped chevron-like grooves has a depth, the device further comprising:crack eliminating means for substantially eliminating cracks in said thin metal strip as said thin metal strip is being cast, said crack eliminating means including means for adjusting at least one of the angular size of said angle defined by said arm portions of said V-shaped chevron-like grooves, said depth of said grooves, said preselected speed of rotation of said at least one roll, and said predetermined speed of rotation of said strip extracting rolls.
 6. Device for casting a thin metal strip, comprising:two cooled substantially parallel rolls rotatable about respective horizontal axes in opposite directions and defining a casting space between said rolls; each roll having a surface on which are provided two series of substantially parallel grooves, said series of grooves each being inclined to the axis of each roll in respective opposite directions, the grooves of one series of grooves meeting respective grooves of the other series of grooves in a transverse plane of each roll so as to form a plurality of chevrons on said surface; said grooves of said rolls facing each other across said casting space; and the two series of grooves forming said plurality of chevrons downwardly converging in a given direction in which said strip is to be cast when said grooves of said rolls face each other across said casting space.
 7. Device according to claim 6, wherein said two substantially parallel rolls are rotatable at a preselected rotational speed, the device further comprising:a cage disposed below said rolls, said cage including a pair of strip extracting rolls which are rotatable at a speed slightly higher than said preselected speed of rotation of said rolls defining said casting space. 